Interdiffusion is the process by which atoms of one metal diffuse into another metal.

• A. Interdiffusion is diffusion of atoms within a single metal
• B. Interdiffusion is diffusion of atoms to a surface
• C. Interdiffusion is diffusion of atoms from one metal into another metal
• D. Interdiffusion is diffusion of vacancies only

Answer: (C)

Interdiffusion describes atoms crossing the boundary between two different metals, moving from one metal into the other to form a mixed region. This differs from diffusion within a single metal (self-diffusion) where atoms move only inside one material, and from diffusion to a surface (surface diffusion) where atoms move along or onto a surface rather than into a different bulk phase. It’s not restricted to vacancies alone, since diffusion can occur via various mechanisms, with vacancies being one common pathway but not the only one.

So the statement that diffusion of atoms from one metal into another best captures interdiffusion.